1. Field of the Invention
The invention relates to systems for detecting the unauthorized removal of objects from a protected area, and in particular, to such systems in which an alternating magnetic field is produced in an interrogation zone, thereby enabling the detection of a ferromagnetic marker.
2. Description of the Prior Art
Antipilferage systems based on the detection of a ferromagnetic marker are well known, having been disclosed at least as early as 1934 in French Pat. No. 763,681 (Picard). Since typical such markers are generally responsive only along an extended dimension, the prior art has recognized that reliable detection will only be achieved with either a multidimensional marker, such as one having long and thin members which are crossed or folded, thereby providing a detectable response to a generally unidimensional interrogating field, or that a multidimensional field or fields must be provided. For example, in U.S. Pat. No. 3,697,996 (Elder and Wright) there is disclosed an apparatus for sequentially producing a plurality of fields, each of which is preferably orthogonal with respect to the other fields at every point in the interrogation zone.
In contrast to such relatively complex systems for ensuring the detection of a unidimensional marker, other systems are known in which a rotating field is provided in the zone such that there is at various times during which a marker is in the zone a field corresponding to all possible orientations of the marker so as to ensure detection thereof at some instant of time during its passage regardless of orientation. See, for example, French Pat. No. 763,681 (Picard) or U.S. Pat. No. 3,990,065 (Purinton et al.). In yet other systems, only a single field is provided in the zone, and the divergence of magnetic fields results in the lines of flux being variously oriented at different regions along a corridor through the interrogation zone. In such a system, the divergence results in different field directions along the corridor so as to improve the detection of the marker at some point along the corridor, regardless of its orientation. See, for example, U.S. Pat. No. 3,820,104 (E. R. Fearon).